The present invention relates to a telephone interface circuit, and particularly relates to improvement of complete cutting off of a telephone line at the time of an internal call.
FIG. 1 shows a circuit configuration for a telephone interface circuit 10 of the related art. The telephone interface circuit 10 controls interfacing between a speech circuit 30 and telephone lines L1, L2. The telephone interface circuit 10 is mainly comprised of a varistor element V1, diode bridge 20, hook switch 40, and transistors Q1, Q2.
The transistor Q1 is a PNP transistor, and the transistor Q2 is an NPN transistor. A resistor R1 is connected across emitter terminal E1 and base terminal B1 of transistor Q1. The emitter terminal E1 of transistor Q1 is connected to a cathode of diode D1, and collector terminal C1 is connected to the speech circuit 30. A resistor R10 is connected across the base terminal B1 of transistor Q1 and a collector terminal C2 of transistor Q2. The emitter terminal E2 of transistor Q2 branches into two, with one branch connected to the speech circuit 30 and the other branch connected to the anode of diode D4. Base terminal B2 of transistor Q2 is connected to a microcomputer (not shown) via resistor R4. The microcomputer (not shown) controls the potential of terminal HC at the time of an on-hook operation, off-hook operation, or dial pulse transmission operation etc. Further, the base terminal B2 of transistor Q2 is connected to terminal 1 of hook switch 40 via resistor R21.
Varistor element V1 absorbs overvoltages (for example, high voltages in the order of 270V or more) across the telephone lines L1, L2.
The diode bridge 20 regulates the signal flowing through the telephone lines L1, L2 for supply to the speech circuit 30. The diode bridge 20 is configured from four diodes D1 to D4.
Hook switch 40 connects terminal 1 and terminal 3 at the time of off-hook. As a result, a forward direction voltage is applied across the base terminal B2 and emitter terminal E2 of the transistor Q2 via resistor R21 and the transistor Q2 therefore turns on. As a result, the base potential of transistor Q1 rises, and the transistor Q1 also turns on. At the time of making a call, the base potential of transistor Q2 is controlled via terminal HC using control of a microcomputer (not shown) so as to correspond to a dial input. As a result, the transistor Q1 sends a dial pulse signal.
Hook switch 40 connects terminal 1 and terminal 2 at the time of on-hook. In doing so, the transistor Q1 disconnects the speech circuit 30 and the telephone lines L1, L2.